In a bleached pulp mill, pulp is formed by digestion of cellulosic fibrous material in a pulping liquor and thereafter is separated from spent pulping liquor. The spent pulping liquor is subjected to recovery and regeneration operations to form fresh pulping liquor for recycle. The pulp is subjected to bleaching and purification in a bleach plant operation. In the bleach plant, treatment chemicals, commonly in aqueous solutions, and wash water are used in the bleaching and purification of the pulp, to provide one or more aqueous bleach plant effluents containing spent chemicals and spent wash water. Such bleach plant effluents usually are discharged, possibly after treatment, to water bodies. The discharge of such bleach plant effluents to water bodies is environmentally hazardous and, hence, objectionable.
Bleach plant operations generally involve bleaching with chlorine, chlorine dioxide, mixtures of chlorine and chlorine dioxide, or other chlorine-containing chemicals and may include oxygen bleaching or delignification, and purification using aqueous sodium hydroxide solution. The pulp usually is washed after each bleaching and/or purification step.
There has previously been proposed in U.S. Pat. No. 3,698,995 one manner of avoiding discharge of the bleach plant effluent or filtrate to water bodies and in this proposal a single liquid effluent is provided from the bleach plant by mixing together the spent aqueous filtrates from each bleach plant operation and the single liquid effluent is used to wash pulp free from spent pulping liquor prior to feed of the pulp to the bleach plant. The bleach plant effluent, after being used in this washing operation is mixed with the spent pulping liquor and passes to the recovery and regeneration operations with the spent pulping liquor, thereby retaining the bleach plant effluent in the mill and avoiding its discharge. Additionally, the prior art fresh water requirement for unbleached pulp washing is decreased by this procedure.
Operation in accordance with the aforementioned U.S. Pat. No. 3,698,995 has proved to give rise to practical difficulties in many cases, resulting from the presence of minor components in the effluent, such as calcium ions, which, in the presence of black liquor solids, may coprecipitate with lignin moities on the pulp, hence returning to the bleach plant and giving rise to increased chemical consumption. Further, insoluble compounds, such as calcium lignin and calcium carbonate, may precipitate on equipment, causing operational difficulties which may lead to downtime for cleaning.
Additionally, organic material dissolved in the purification stages and present in the combined effluent will not be fully removed from the pulp in the washing before the pulp returns to the bleach plant, again increasing chemical consumption.
Moreover, the volume of the combined effluent may be greater than that required for the washing, thereby increasing the evaporative load on the black liquor evaporator.